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Transcript
Chapter 5 Section I: The Cell and In Its Environment Introduction All cells are surrounded by a cell membrane that separates the cells from the outside environment. Cells have structures that protect them from the outside environment. The cell has to take in needed materials and get rid of waste. The cell membrane controls what goes in and out of the cell. The Cell Membrane as a Gatekeeper The selectively permeable (able to pass through) structure of the cell membrane enables it to regulate the materials that enter and leave the cell. A cell membrane is usually permeable to substances such as oxygen, water, and carbon dioxide. The Cell Membrane as a Gatekeeper Substance move in and out of the cell in one of three methods: 1. 2. 3. Diffusion Osmosis Active Transport Diffusion – Molecules in Motion Diffusion is the main method in which small molecules move into and out of cells. The process by which molecules move from higher concentration to an area of lower concentration. (Crowded Room to Less Crowded Room) The concentration of a substance is the amount of the substance in a given volume. (How Strong) What causes diffusion? Molecules are always moving. Collisions cause molecules to push away from one another. The more molecules, the more collisions that occur. Naturally, they will spread out evenly. Diffusion of Cells In the pond water there are many more molecules of oxygen in the water than in the cell. The structure of the cell membrane is permeable to oxygen molecules. The oxygen molecules diffuse from an area of higher concentration to an area of lower. Osmosis – The Diffusion of Water Molecules The Diffusion of water molecules through a selectively permeable membrane is called osmosis. Osmosis is important to sustain life because cells cannot function properly without adequate water. In osmosis, water molecules move by diffusion from an area where they are highly concentrated through the cell membrane to an area where they are less concentrated. Osmosis – The Diffusion of Water Molecules Red blood cells are bathed in a solution in which concentration of water is the same as it is on the inside of the cells. Salt water pulls water from the blood cells. Active Transport Passive Transport = Is the movement of materials through a cell membrane without using energy. Active Transport = is the movement of materials through a cell membrane using energy. The main difference between passive transport and active transport is that active transport requires the cell to use energy while passive transport does not. Transport Proteins Transport proteins are located in the cell membrane. Transport proteins pick up molecules outside the cell and carry them in using energy. Transport proteins carry some substances in and out of cells: Calcium, Potassium Sodium. Transport By Engulfing First the cell membrane surrounds, or engulfs, a particle. Once the particle is engulfed, the cell membrane pinches off and forms a vacuole within the cell. The cell must use energy in this process. Why Are Cells So Small One reason is related to how materials move into and out of cells. As a cell’s size increases, more of its cytoplasm is located farther from the cell membrane. Once a molecule enters a cell, it is carried to its destination by a stream of moving cytoplasm. But in a very large cell, the streams of cytoplasm must travel farther to bring materials to all parts of the cell = more time for to complete tasks and remove wastes. Why Are Cells So Small If a cell grew too large, it could not function well enough to survive. When a cell reaches a certain size, it divides into two new cells.